The Old Prussians or Baltic Prussians (German: Pruzzen or Prußen; Latin: Pruteni; Latvian: Prūši; Lithuanian: Prūsai; Polish: Prusowie) were an ethnic group, autochthonous Baltic tribes that inhabited Prussia, the lands of the southeastern Baltic Sea in the area around the Vistula and Curonian Lagoons. They spoke a language now known as Old Prussian and followed pagan Prussian mythology.

During the 13th century, the Old Prussians were conquered by the Teutonic Knights, and gradually assimilated over the following centuries. The former German state of Prussia took its name from the Baltic Prussians, although it was led by Germans who had assimilated the Old Prussians; the old Prussian language was extinct by the 17th or early 18th century.

The land of the Old Prussians consisted approximately of central and southern East Prussia — the present-day Warmian-Masurian Voivodeship of Poland, the Kaliningrad Oblast of Russia, and the southern Klaipėda Region of Lithuania.

The names of the Baltic Prussian tribes all reflected the theme of landscape. Most of the names were based on water, an understandable convention in a land dotted with thousands of lakes, streams, and swamps (the Masurian Lake District). Indeed, that landscape caused the very partial isolation that preserved the Baltic language group as the most archaic in Europe. To the south, the terrain runs into the Pripet Marshes at the headwaters of the Dnieper River; these have been an effective barrier over the millennia.